Nanoscale diffusion, compound formation and phase transitions in Mo/Si multilayer structures

This thesis addresses the physical and chemical phenomena in Mo/Si multilayer structures with and without B4C diffusion barrier layers at the interfaces, which are applied as extreme ultraviolet (EUV) / soft X-ray optics. Since interdiffusion and interlayer formation limit both the performance and the lifetime of the optics, the interlayers and their evolution in time have been investigated in detail. To this end, a procedure was developed to utilize the depth-resolved information in a LEIS spectrum in order to non-destructively study interdiffusion in ultrathin films with sub-nanometer resolution. Using this method, it was found that the diffusion in Mo/B4C/Si layered structures initially obeys Fick’s second law. However, after a certain time, the diffusion rate instantaneously increased by up to one order of magnitude. The cause of this acceleration was found to be the amorphous-to-nanocrystalline transition of the MoSi2 interface. Besides diffusion and crystallization, the chemical processes upon annealing of Mo/B4C/Si layered structures have been identified. Mo/B/Si and Mo/C/Si samples were studied as reference systems. Initially, predominantly MoBxCy (resp. MoBx, MoCx) formed, plus small amounts of SiBxCy (resp. SiBx, SiCx). Subsequently, MoSi2 formed, while the already formed MoBxCy (resp. MoBx, MoCx) diffused further into the Mo layer. Stable silicides, through which Si diffused to form MoSi2 in the second stage, only formed in the samples with C and B4C interlayers. Furthermore, it was investigated whether the interlayer thickness can be reduced by depositing the multilayer mirror at a low temperature. Even after warming up to room temperature, the interlayers that formed upon cryogenic deposition were found to be nearly 60 % thinner than after room temperature deposition. Finally, since rough interfaces between the layers of a multilayer mirror are equivalent to thick interlayers, it is important to control the roughness during the deposition. The development of roughness is usually mitigated through ion bombardment of the Si layers. Therefore, in order to gain understanding, the roughness evolution of Si surfaces upon Ar ion erosion was studied in real-time. It was demonstrated that ion treatment can cause roughening as well as smoothening, depending on the initial roughness of the substrate.